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Sometimes men wake up with an erection in the morning. Why does this happen?

Shortly speaking: REM (Rapid Eye Movement) is one phase of sleep. During this phase we dream and some of our neurotransmitter are shut off. This include norepinephrine, which is involved in controlling erection. Norepinephrine prevents blood to enter the penis (preventing the erection). In absence of norepinephrine (during REM phase norepinephrine is absent) blood enters the penis yielding to an erection. This phenomenon is called Nocturnal penile tumescence. Such erections typically occurs 3 to 5 times a night. A related question on the concern of women erections can be found here.

A high pressure in the bladder may also lead to a "reflex erection". This erection allows to prevent uncontrolled urination. The drawback is that when, in the morning one has an erection and can't wait to pee it might get hard to accurately target the toilets!

This video is also a nice and easy source of information on the subject.

Is it bad? (Reading my "note" below, you have edited your post to get rid of this question, thank you)

It is perfectly healthy you don't have to worry about that. These erections are even thought as contributing to penile health. At the opposite, the absence of erections during the night are an index of Erectile Dysfunction (E.D.).

Note

Be aware for future posts that medical questions are often considered as off-topic on this site. Asking "is it bad?" turns your question into a medical one.

I just had to, had to signup to this site for the sake of upvoting this... man, this is crazy...
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Awal GargJul 23 '14 at 11:45

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How does the lack of norepinephrine affect women?
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jnovachoJul 23 '14 at 12:03

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That sound's great... How do I permanently shutoff this norepinephrine?? :-D
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ThiharaJul 23 '14 at 12:05

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@Thihara Norepinephrine affects a lot more than just erections. Killing it from your system entirely would be a very bad idea. (Just FTR - I know, or at least really hope, your question was intended as sarcasm.)
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IsziJul 23 '14 at 18:48